1. Field of the Invention
The invention is directed to a novel cathode construction for an all-solid state lithium electrochemical cell and a method of forming the cathode. In particular, the invention is directed towards the cathode layer of a multilayer electrochemical lithium cell having a polymeric electrolyte layer a cathode layer containing vanadium oxide, and a lithium or lithium/aluminum alloy anode layer.
2. DESCRIPTION OF THE PRIOR ART
Electrochemical cells and batteries have been constructed from a wide variety of materials. Both the electrodes and the dielectric layer have been made from plastic, metal, and other substances. The electrolyte has usually been a liquid but solid material, such as polymers, are now preferred in lithium battery construction.
A solid state battery will avoid many of the problems commonly associated with liquid electrolyte cells. Such problem sinclude electrolyte leakage, dryout, anode passivation, and dendrite formation. In addition, the use of all solid state components simplifies fabrication of the cell and leads to a mechanically stable device. Operation at moderate temperature overcomes the severe problems of corrosion and sealing associated with high temperature fused salt electrolyte or molten electrode systems. Clearly a solid state battery is preferable for many applications to a battery containing liquid electrolyte or electrolyte paste.
It has been preferred when fabricating lithium batteries to use a polymer/inorganic composite as both the cathode layer and the electrolyte layer, which layers may be formed as a film by continuous casting and solvent evaporation. Using this method, large area membranes of 25-50 micrometer thickness may be routinely fabricated. This technique, commonly referred to as the "doctor-blade" technique, results in electrolyte layers which are substantially pinhole-free, retain their integrity over many cycles, and provide excellent interfacial characteristics.
Recent joint studies conducted by the Harwell Laboratory and the Energy Research Laboratory of Odense University have focused upon lithium batteries employing a polymer electrolyte composed of polyethylene oxide compounded with various lithium salts. In these cells, the cathode material is based on V.sub.6 O.sub.13 and the preferred anode consists of a lithium metal foil or a lithium/aluminum foil. Specifically, the cathode is a composite structure formed by intimately mixing pre-milled vanadium oxide with acetylene black, in an electrolyte solution. The electrolyte solution contains polyethylene oxide polymer. The resulting cathode layer, when deposited as a film, consists of a random agglomerate of particles of polymer, carbon, and vanadium oxide. It is reported that the lithium cells so constructed showed improved performance in terms of current density, material utilization, and reproduceabilty. It is suggested that these cells may have application in the production of vehicle traction batteries. See, Hooper, A. et al., Advanced Battery Development (Odense University Press, 1984).